Iridium-catalyzed asymmetric hydrogenation of racemic α-substituted lactones to chiral diols

Article abstract of DOI:10.1039/c6sc04609f

We report a protocol for the highly efficient iridium-catalyzed asymmetric hydrogenation of racemic α-substituted lactones via dynamic kinetic resolution. Using Ir-SpiroPAP (R)-1d as a catalyst, a wide range of chiral diols were prepared in a high yield (80-95%) with a high enantioselectivity (up to 95% ee) under mild reaction conditions. This protocol was used for enantioselective syntheses of (?)-preclamol and a chiral 2,5-disubstituted tetrahydropyran.

Full text of DOI:10.1039/c6sc04609f

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Iridium-Catalyzed Asymmetric Hydrogenation of Racemic α-
Substituted Lactones to Chiral Diols
Xiao-Hui Yang,a Hai-Tao Yue,a Na Yu,a Yi-Pan Li,a Jian-Hua Xie*a and Qi-Lin Zhou*ab
Received 00th January 20xx,
Accepted 00th January 20xx
We report a protocol for highly efficient iridium-catalyzed asymmetric hydrogenation of racemic α-substituted lactones via
dynamic kinetic resolution. With Ir-SpiroPAP (R)-1d as a catalyst, a wide range of chiral diols were prepared in high yield
(80−95%) with high enantioselectivity (up to 95% ee) under mild reaction conditions. The protocol was used for
DOI: 10.1039/x0xx00000x
www.rsc.org/
enantioselective
syntheses
of
(−)-preclamol
and
a
chiral
2,5-disubstituted
tetrahydropyran.
also mediate the hydrogenation of ester groups.7
Introduction
1) Ikariya's work (2011)
Transition-metal-catalyzed asymmetric hydrogenation of
ketones is an efficient and reliable method for the synthesis of
optically active chiral secondary alcohols.1 In contrast, the
enantioselective synthesis of chiral primary alcohols by
catalytic asymmetric hydrogenation of the corresponding
aldehydes or esters is difficult, and work on the development
of practical methods is underway. In 2007, we reported the
first example of catalytic asymmetric hydrogenation of racemic
O
OH
OH
2 mol% Ru-cat*
Ph
Ph
H2
O
+
tBuOK, iPrOH
80 oC, 48 h
Ru
NMe2
Cl
(9.8 atm)
82% yield
32% ee
Me2N
Ru-cat*
2) This work
OH
O
0.2 mol% (R)-1d
α
-branched aldehydes via dynamic kinetic resolution (DKR) for
3
R
R
O
H2
the synthesis of chiral primary alcohols.2 Subsequently, List
+
tBuOK, nPrOH
rt, 7−36 h
OH
( )n
( )n
4
and Lin et al. also reported the synthesis of chiral primary
(10 atm)
n = 1 and 2
R = alkyl, aryl
80−95% yield
69−95% ee
alcohols, by means of ruthenium-catalyzed asymmetric
hydrogenation of racemic α-substituted aldehydes. Although a
wide range of catalysts have been developed for the
hydrogenation of esters,5 efficient chiral catalysts for
ꢀHigh enantioselectivity
ꢀLow catalyst loading
ꢀMilder hydrogenation condition
( )-1a (Ar = C H , X = H)
R
Cl
Ar2
P
H
N
asymmetric hydrogenation of racemic
α-substituted esters via
6
5
Ir
(R)-1b (Ar = 3,5-(Me)2C6H3, X = H)
)-1c (Ar = 3,5-(tBu) C H , X = H)
DKR are rare. The biggest challenge for direct asymmetric
hydrogenation of racemic esters to form optically active
primary alcohols is to find a catalyst that can discriminate the
enantiomers of chiral esters and hydrogenate them to alcohols
X
(
R
N
H
2
6 3
H
(R)-1d (Ar = 3,5-(tBu)2C6H3, X = 3-Me)
(R)-1e (Ar = 3,5-(tBu)2C6H3, X = 4-tBu)
(R)-1f (Ar = 3,5-(tBu)2C6H3, X = 6-Me)
Ir-(R)-SpiroPAP((R)-1)
selectively. In 2011, Ikariya et al.6 described the Scheme 1 Asymmetric hydrogenation of racemic
α-substituted lactones via DKR.
enantioselective hydrogenation of a racemic
α-substituted γ-
In this communication, we report a protocol for Ir-SpiroPAP-
catalyzed asymmetric hydrogenation of racemic -substituted
95%) with high
lactones to chiral 1,4-diol via DKR using chiral ruthenium
catalysts bearing chiral 1,2-diamine ligands (Scheme 1).
However, the enantioselectivity of the reaction was low (up to
32% ee). Recently, as part of our work on the asymmetric
hydrogenation of ketones, we found that chiral Ru-
SDPs/diamine catalysts and chiral Ir-SpiroPAP catalysts can
α
lactones to afford chiral diols in high yield (80
−
enantioselectivity (up to 95% ee, Scheme 1).
Results and discussion
We initially performed the hydrogenation of racemic α-phenyl
-valerolactone 2a to evaluate the activity and
δ
(
)
enantioselectivity of various catalysts (Table 1). Under
previously reported reaction conditions,7b that is, catalyst
loading = 0.2 mol % (S/C = 500), [2a] = 0.25 M, [tBuOK] = 0.04
M, EtOH, 10 atm H2, 25
−
30 °C, no reaction occurred in the
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presence of catalyst (R)-1d (entry 1). However, when the much effect on the enantioselectivity of the reaction, but
concentration
substrates with an electron-withdrawing group (entries 2, 5,
and 8) showed a higher reaction rate than those with an
electron-donating group.
Table 1 Asymmetric hydrogenation of 2a. Optimizing reaction conditions.a
Table 2 Asymmetric hydrogenation of racemic α-substituted lactones to chiral diols
with (R)-1d.a
Time Yield
Ee
Entry
Cat.
Base
[base]
Solvent
(h)
24
24
10
10
10
10
17
10
7
(%)b (%)c
1
2
(R)-1d
(R)-1d
(R)-1d
(R)-1a
(R)-1b
(R)-1c
(R)-1e
(R)-1f
(R)-1d
(R)-1d
(R)-1d
(R)-1d
(R)-1d
(R)-1d
(R)-1d
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuONa
KOH
0.04
0.06
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.25
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
nPrOH
iPrOH
nPrOH
nPrOH
nPrOH
nPrOH
ND
15
91
93
92
92
91
92
10
92
89
91
48
36
21
ND
91
92
90
90
91
90
90
86
93
74
93
93
93
92
Time
(h)
10
7
Yield
(%)b
92
93
92
93
95
91
93
94
91
89
84
88
92
90
90
91
80
82
Ee
(%)c
93 (R)
93
Entry
R
n
3
3
4
1d
2
C6H5
4-ClC6H4
4-MeC6H4
4-MeOC6H4
3-ClC6H4
3-MeC6H4
3-MeOC6H4
3,4-Cl2C6H3
3,4-(MeO)2C6H3
2-ClC6H4
2-MeC6H4
2-MeOC6H4
Me
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
1
3a
3b
3c
3d
3e
3f
5
6
3
9
93
7
4
10
7
93
8
5
92
9
6
10
10
7
93
10
11
12
13
14
15
10
8
7
8d
3g
3h
3i
92 (R)
92
10
20
20
20
9
10
13
36
20
8
91
10
11
12
13d
14
15d
16d
17
18
3j
78
NaOH
K2CO3
3k
3l
77
86
a Reaction conditions: 1.0 mmol scale, [2a] = 0.25 M, 0.2 mol% of catalyst, solvent
3m
3n
3o
3p
3q
3r
91 (S)
87
b
c
(4.0 mL), room temperature (25−30 °C). Isolated yield. Determined by HPLC
using chiral column. The absolute configuration of 3a is R determined by
comparison of its optical rotation with the literature date (see Supporting
Information).
Et
iPr
12
12
12
10
10
95 (R)
88 (S)
80 (R)
69 (S)
CH2=CHCH2CH2
C6H5
Me
a Reaction conditions: 1.0 mmol scale, [substrate] = 0.25 M, [tBuOK] = 0.25 M, 0.2
of tBuOK was increased to 0.06 M, the hydrogenation reaction
occurred and provided (R)-3a in 15% yield with 91% ee (entry
2); and further increasing the concentration of tBuOK
increased the reaction rate and the yield of 3a substantially.
c
mol% of (R)-1d, nPrOH (4.0 mL), room temperature (25−30 °C). b Isolated yield.
d
Determined by HPLC using chiral column. The absolute configuration of the
product is determined by comparison of their optical rotation with the literature
data (see Supporting Information)
For example, when 0.25
500:500:1) was used, the reaction was complete within 10 h,
M ( =
tBuOK 2a/tBuOK/(R)-1d
providing (R)-3a in 91% yield with 92% ee (entry 3). The In addition, owing to steric effects, substrates with an ortho-
absolute configuration of (R)-3a was determined by substituent on the phenyl ring gave lower reaction rates, yield,
comparison of the sign of its optical rotation with literature and enantioselectivity (entries 10
data.8 Evaluation of various chiral Ir-SpiroPAP catalysts (R)-
-alkyl-substituted -valerolactones 2m-p also worked well,
revealed that the substituents on the pyridine and phenyl affording corresponding 1,5-diols 3m-p in high yield and high
groups of the catalysts had little effect on the yield or enantioselectivity (entries 13
16). Catalyst (R)-1d also
enantioselectivity (entries 4–8), with (R)-1d giving the best catalyzed the hydrogenation of racemic
-substituted
−12). The hydrogenation of
1
α
δ
−
α
γ-
n
results. Experiments with various solvents showed that PrOH butyrolactones 2q and 2r, providing 1,4-diols 3q (80% ee) and
was suitable (entries 9–11): the reaction was complete within 3r (69% ee), respectively (entries 17 and 18).
10 h, affording (R)-3a in 92% yield with 93% ee. In addition to
tBuOK, tBuONa also gave
a
high yield with high
enantioselectivity, but the use of KOH, NaOH, or K2CO3
resulted in low yields (entries 12–15).
To evaluate the substrate scope of the reaction, we
investigated
valerolactones under the established reaction conditions
(Table 2). For racemic -aryl-substituted -valerolactones 2a-i
(entries 9), neither electron-donating nor electron-
a wide range of racemic α-substituted δ-
α
δ
1−
withdrawing groups on the phenyl ring of the substrates had
2 | J. Name., 2012, 00, 1-3
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Scheme 2 Asymmetric hydrogenation of ester
4
and
5
with (R)-1d.
OH
OH
O
H2 (10 atm)
0.2 mol% (S)-1d
MeO
MeO
O
tBuOK, nPrOH, 25−30 oC,10 h
89%
rac-2g
1.65 g (80 mmol)
(S)-3g
(93% ee)
1) MsCl, NEt3
DCM, −20 oC, 2 h
OH
2) nPrNH2, rt, 15 h
3) 48% HBr, 120 oC, 2 h
N
Fig. 1 The plots of the hydrogenation of rac-2a with (R)-1d.
1.3 g
84% (3 steps)
( )-preclamol
−
We investigated the pathway of the hydrogenation of
1
by H NMR. As shown in Fig.
Scheme 3 Enantioselective synthesis of (−)-preclamol.
racemic
α
-substituted lactones
2
1, after reaction for 0.5 h under the optimal reaction
conditions, rac 2a was converted to hydroxyl ester , propyl 5-
-
4
HO
HO
O
HO
I2, NaHCO3
NaCN
I
hydroxy-2-phenylpentanoate, in 62.3% yield with no ee and
CH3CN, 0 o
C
DMSO, 80 oC, 3 h
91%
diol 3a in 37.7% yield with 93% ee. In the following 10 h, the
to rt, 2 h
94%
(
S
)-3p (88% ee)
7 (
/ = 2:1)
trans cis
hydroxyl esters
4 gradually decreased, and the diol 3a
increased. Only trace amount of lactone 2a was detected from
2 min after reaction started.
1) NaOH, EtOH/H2O
(3:1); reflux, 20 h
O
O
NC
MeO2C
Direct hydrogenation of hydroxyl ester
4 with catalyst (R)-1d
2) MeOH, H2SO4
reflux, 20 h
OH
OH
provided diol 3a in 93% yield with 93% ee, which is the same
as the result obtained from the hydrogenation of lactone 2a
(Scheme 2). We also conducted the hydrogenation of the ester
79% (2 steps)
8 (trans/cis = 2:1)
9 (trans/cis = 5:1)
5
, which has a
hydroxyl ester
indicated that the hydroxyl group of ester
hydrogenation. Thus, although lactone 2a was readily
alcoholized to hydroxyl ester under the reaction conditions,
δ
-OCH2OMe group instead of a
, and observed no reaction. These results
is crucial for the
δ-OH group as in
O
HO2C
4
4
(+)-rhopaloic acid A
4
O
the hydrogenation of 2a occurred inevitably via lactone form
(Scheme 2).7b,c
HO2C
OH
Chiral 3-aryl/alkyl substituted piperidines are of an
important class of bioactive heterocyclic compounds,9 but they
are difficult to synthesize in optically active form.10 By using
catalyst
(+)-barangcadoic acid A
Scheme
tetrahydropyran.
4
Enantioselective synthesis of
a
chiral 2,5-disubstituted
O
Ph
OH
(S
)-1d, we synthesized (−)-preclamol,11 a candidate drug for
treating neurological disorders such as Parkinson’s disease.12
The hydrogenation of rac 2g (1.65 g) catalyzed by ( )-1d
afforded diol )-2g (89% yield, 93% ee), which was
subsequently transformed to (−)-preclamol by activation with
methanesulfonyl chloride, substitution/cyclization with
Ph
0.2 mol% (R)-1d
OnPr
OH
H2
+
tBuOK, nPrOH
25−30 oC,10 h
OH
(10 atm)
-
S
(R)-3a
4
93% yield, 93% ee
(
S
O
O
n
-
tBuOK, nPrOH
Ph
H2/(R)-1d
propylamine, and demethylation with hydrobromic acid (84%
yield over three steps, Scheme 3).
Diol (S)-3p is a useful building block for the synthesis of
2a
chiral 2,5-disubstituted tetrahydropyrans, which occur in many
biologically active natural products such as the terpenoids
rhopaloic acid A13 and barangcadoic acid A14 (Scheme 4),
O
Ph
0.2 mol% (R)-1d
Ph
OH
OnPr
H2
+
´
tBuOK, nPrOH
isolated from marine sponges. Iodoetherification of (
iodine15 produced tetrahydropyran
in 94% yield as a 2:1
trans cis mixture. Nucleophilic substitution of tetrahydropyran
with NaCN afforded a nitrile trans cis = 2:1). Hydrolysis of
S)-3p with
OMOM
25−30 oC,10 h
(10 atm)
OMOM
7
6
5
/
no reaction
7
8
(
/
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Vries, C. J. Elsevier), Wiley-VCH: Weinheim, 2007, p 1105; (c)
H. Pellissier, Tetrahedron, 2011, 67, 3769.
X. Li and B. List, Chem. Commun. 2007, 1739.
the nitrile
8
and subsequent esterification with MeOH afforded
in 72% yield with a higher trans cis ratio
tetrahydropyran
9
/
3
4
(5:1). Thus, this protocol represents a potential method for the
construction of the chiral core structure of rhopaloic acid A
and barangcadoic acid A.16
D. J. Mihalcik and W. Lin, Angew. Chem., Int. Ed., 2008, 47
6229.
,
5
For reviews on catalytic hydrogenation of esters, see: (a) M.
L. Clarke and G. J. Roff, in The Handbook of Homogeneous
Hydrogenation, (Eds.: J. G. de Vries and C. J. Elsevier), Wiley,
New York, 2007, p 413; (b) M. Ito, T. Ikariya, Chem.
Commun., 2007, 5134; (c) P. A. Dub and T. Ikariya, ACS Catal.,
Conclusions
In conclusion, we have developed a protocol for highly
efficient iridium-catalyzed asymmetric hydrogenation of
2012, 2, 1718; For selected recent papers, see: (d) J. Zhang,
G. Leitus, Y. Ben-David and D. Milstein, Angew. Chem., Int.
Ed., 2006, 45, 1113; (e) L. A. Saudan, C. M. Saudan, C.
Debieux and P. Wyss, Angew. Chem., Int. Ed., 2007, 46, 7473;
(f) D. Spasyuk, S. Smith and D. G. Gusev, Angew. Chem., Int.
Ed., 2012, 51, 2772; (g) D. Spasyuk, S. Smith and D. G. Gusev,
Angew. Chem., Int. Ed., 2013, 52, 2538.
racemic
α
catalyst, a series of racemic
-substituted lactones via DKR. Using Ir-SpiroPAP
-substituted lactones were
α
hydrogenated to chiral diols in high yield with high
enantioselectivity under mild reaction conditions. The protocol
was used for enantioselective syntheses of (−)-preclamol and a
chiral 2,5-disubstituted tetrahydropyran.
6
7
M. Ito, T. Ootsuka, R. Watari, A. Shiibashi, A. Himizu and T.
Ikariya, J. Am. Chem. Soc., 2011, 133, 4240.
(a) C. Liu, J.-H. Xie, Y.-L. Li, J.-Q. Chen and Q.-L. Zhou, Angew.
Chem., Int. Ed., 2013, 52, 593; (b) X.-H. Yang, J.-H. Xie, W.-P.
Liu and Q.-L. Zhou, Angew. Chem., Int. Ed., 2013, 52, 7833;
(c) X.-H. Yang, K. Wang, S.-F. Zhu, J.-H. Xie and Q.-L. Zhou, J.
Am. Chem. Soc., 2014, 136, 17426.
Experimental
General procedure for asymmetric hydrogenation of 2
20 mL hydrogenation vessel in an autoclave was added
racemic -substituted δ-valerolactone (1.0 mmol), a solution
of iridium catalyst ( )-1d in PrOH (dried with MS 4Å for 12 h,
：To a
8
9
S. S. M. A. Hakim and T. Sugimura, Org. Lett., 2010, 12, 3626.
(a) M. J. Schneider, Alkaloids: Chem. Biol. Perspect., 1996, 10
155; (b) R. J. Andersen, R. W. M. Van Soest and F. Kong,
Alkaloids: Chem. Biol. Perspect., 1996, 10 301; (c) L.
,
α
2
n
,
R
t
Cervetto, G. C. Demontis, G. Giannaccini, B. Longoni, B.
Macchia, M. Macchia, A. Martinelli and E. Orlandini, J. Med.
Chem., 1998, 41, 4933; (d) M. Macchia, L. Cervetto, G. C.
Demontis, B. Longoni, F. Minutolo, E. Orlandini, G. Ortore, C.
0.002 mmol/mL, 1.0 mL, 0.002 mmol), a solution of BuOK in
n
nPrOH (0.5 mmol/mL, 2.0 mL, 1.0 mmol) and PrOH (1.0 mL).
The autoclave was purged with hydrogen by pressurizing to 5
atm and releasing the pressure. This procedure was repeated
Papi, A. Sbrana and B. Macchia, J. Med. Chem., 2003, 46
161.
,
three times and then pressurized to 10 atm of H2. The reaction
o
30 C) until no
10 F. Colpaert, S. Mangelinckx and N. D. Kimpe, J. Org. Chem.,
2011, 76, 234 and references therein.
mixture was stirred at room temperature (25
−
obvious hydrogen pressure drop was observed. The reaction
mixture was then quenched with saturated NH4Cl (5 mL) and
11 For selected papers on asymmetric synthesis of preclamol,
see: a) J. Hu, Y. Lu, Y. Li and J. Zhou, Chem. Commun., 2013,
49, 9425; (b) Z. Huang, Z. Chen, L. H. Lim, G. C. P. Quang, H.
Hirao and J. Zhou, Angew. Chem., Int. Ed., 2013, 52, 5807; (c)
J, Y. Hamilton, D. Sarlah and E. M. Carreira, Angew. Chem.,
Int. Ed., 2015, 54, 7644; (d) T. Jia, P. Cao, B. Wang, Y. Lou, X.
extracted with EtOAc (5 mL
× 3). The combined extracts were
washed with brine, dried over anhydrous MgSO4 and
concentrated in vacuo. The residue was purified by flash
column chromatography on silical gel with petroleum
Yin, M. Wang and J. Liao, J. Am. Chem. Soc., 2015, 137
13760.
,
ether/ethyl acetate as an eluent to afford the chiral diols 3.
The ee values of the chiral diols
using chiral column.
3 were determined by HPLC
12 (a) U. Hacksell, L. E. Arvidsson, U. Svensson, J. L. G. Nilsson,
D. Sanchez, H. Wikstrom, P. Lindberg, S. Hjorth and A.
Carlsson, J. Med. Chem., 1981, 24, 1475; (b) C. Sonesson, C.-
H. Lin, L. Hansson, N. Waters, K. Svensson, A. Carlsson, M. W.
Smith and H. Wikstrom, J. Med. Chem., 1994, 37, 2735.
13 (a) S. Ohta, M. Uno, M. Yoshimura, Y. Hiraga and S. Ikegami,
Tetrahedron Lett., 1996, 37, 2265; (b) M. Yanai, S. Ohta, E.
Ohta and S. Ikegami, Tetrahedron, 1998, 54, 15607.
Acknowledgements
Project supported by the National Natural Science Foundation
of China, the National Basic Research Program of China (973
Program) (No. 2012CB821600), the “111” project (No. B06005)
of the Ministry of Education of China.
14 K. S. Craig, D. E. Williams, I. Hollander, E. Frommer, R.
Mallon, K. Collins, D. Wojciechowicz, A. Tahir, R. V. Soest and
R. J. Andersen, Tetrahedron Lett., 2002, 43, 4801.
15 L. F. Tietze and C. Schneider, J. Org. Chem., 1991, 56, 2476.
16 For selected papers on the synthesis of rhopaloic acid A and
its analogues, see: (a) K. Kadota and K. Ogasawara,
Heterocycles, 2003, 59, 485; (b) J. C. R. Brioche, K. M.
Goodenough, D. J. Whatrup and J. P. A. Harrity, Org. Lett.,
Notes and references
1
(a) J. G. de Vries and C. J. Elsevier, Eds. The Handbook of
Homogeneous Hydrogenation; Wiley-VCH: Weinheim, 2007;
(b) B. Zhao, Z. Han and K. Ding, Angew. Chem., Int. Ed., 2013,
52, 4744; (c) J.-H. Xie and Q.-L. Zhou, Acta. Chim. Sinica,
2014, 72, 778.
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2
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